We have described four yeast RNA replicons, namely the L-A and L-BC dsRNA viruses, and the ssRNA replicons, 2OS RNA and 23S RNA. The propagation of all four of these replicons is partially blocked by the SKI2, SKI3, SKI4, SKI6, SKI7 and SKI8 genes, which we have shown act by blocking the translation of poly(A) mRNAs, such as the viral messages. Two close human homologues of SKI2 have been described. one of which has been shown to be a nucleolar protein. We have found that the concentration of free 6OS ribosomal subunits is critical for L-A virus propagation. probably because of a role of the 3' poly(A) structure of the mRNA in attracting and activating 60S subunits for translation. We have previously characterized the SKI2, 3, and 8 genes, and we have now cloned the SKI6 and SKI7 genes. Their sequencing and characterization will further define this important system. We find that the L-A virus has an icosahedral structure with T=1 and an assymetric unit comprised of a dimer of Gag, the major coat protein. Preliminary studies by others indicate that the core of reovirus and rotavirus have essentially the same structure. We are studying the transcriptase activity of the L-A virus. using our template dependent system. We find that this enzyme recognizes a signal within the first l8 bp of the L-A dsRNA structure. We have also detected the RNA-dependent RNA polymerases of 2OS RNA and of the L-BC virus and are studying their specificities.